1. Field of the Invention
The present invention relates generally to the field of telecommunications, and more particularly, the present invention relates to an improved co-channel suppression system for use in radio telecommunications.
2. Description of the Prior Art
In many radio systems, available frequencies are re-used in adjacent geographic areas to provide increased traffic capacity for a given bandwidth. Frequency use, however, is fundamentally limited by the tolerance of a receiver to co-channel interference. Although transmission formats largely dictate the extent of this tolerance, techniques such as antenna diversity reception, adaptive power control or adaptive antennas may be used to improve upon this. Adaptive and diversity antennas have limited application, however, and ideally the suppression of co-channel interference using the signal from a single antenna is therefore desirable.
The level of interference that a receiver can tolerate fundamentally limits system capacity because frequencies (or channels) are re-used spatially. The more interference a receiver can tolerate, the greater the proportion of available frequencies which can be used at each base station. Receivers in land based mobile radio systems are rarely designed to account for the presence of interference, rather the system is configured to reduce the level of interference experienced to an acceptable level. If, however, the detection processes in a receiver are modified explicitly to account for the presence of a number of like modulated co-channel interfering signals, then the receiver can potentially tolerate a much greater level of co-channel interference and the system capacity can therefore advantageously be increased.
It is an object of the present invention therefore to provide a system which will operate to suppress similar digitally modulated co-channel interference using the signal from a single antenna.
It is a further object of the present invention to provide a system which can be applied to an existing system with minimal modification.
According to the present invention, in a communication system wherein known data sequences characterize both wanted transmitted signals and interfering co-channel transmitted signals, a receiver for receiving a wanted transmitted signal in the presence of unwanted co-channel interfering signals comprises a signal sequence identifier means which includes a memory for storing known data sequences and a correlator which serves to correlate received signals with the stored known data sequences. The relative location in time of received signal sequences is thus identified. An impulse response estimator estimates the impulse response of each signal, the location of which has been identified, and a means responsive to the estimated impulse responses produces a set of possible signal configurations in respect of each said response. A detector means includes a means for comparing the signals of each set with the received signal to thereby process the desired signal.
The sequence identifier may be fed with the received signal via a baseband signal converter and an A-to-D converter which in combination serve to provide digital signal samples of the received signal at baseband frequencies.
The means for comparing the signals of each set with the received signal may comprise a metrics generator responsive to the sets for providing signals for a sequence estimator comprising a trellis processor from which an output signal comprising a detected data sequence corresponding to the wanted signal is provided.
The baseband signal converter may be fed with a signal from a single antenna and may be used instead of a demodulator/equalizer in an existing system to thereby provide improved co-channel interference suppression with minimal modification to the existing system.